The present invention relates to anaesthetic vaporisers, which are devices for mixing the vapour of a volatile liquid anaesthetic agent with a supplied gas (which term is to be understood to include gas mixtures), for subsequent administration to a patient. Throughout this specification, the term `anaesthetic` is intended to embrace both anaesthetics and analgesics.
Particularly, though not exclusively, the invention is concerned with anaesthetic vaporisers of the by-pass type wherein the gas supplied to the vaporiser is divided into two streams, the first of which passes through a vaporising chamber where the gas becomes saturated with the vapour of the volatile agent and the second of which by-passes the vaporising chamber, the two streams subsequently reuniting. The vaporising chamber usually comprises a sump for liquid anaesthetic over which the supplied gas can flow, usually also with wicks extending into the liquid and past which the gas also flows.
In normal usage this by-pass arrangement permits the concentration of anaesthetic vapour in the gas delivered from the vaporiser to be controlled to a high degree of accuracy and reliability. Specifically, this may be achieved by means of a concentration dial linked to, or forming part of, a valve which can be adjusted to control the resistance to gas flow through the vaporising chamber circuit. This has the effect of altering the proportions in which the inflowing gas is divided into the two aforementioned streams, hence altering the concentration of vapor-saturated gas in the subsequently reunited flow. Furthermore, a thermal compensation valve may be employed in the by-pass circuit by which the resistance to gas flow through that circuit can be controlled. This again affects the proportioning of the two streams and the parameters of this valve can be selected so as substantially to compensate for variations in the vapour pressure to the volatile agent (and hence its saturation concentration) with temperature.
Nevertheless, anaesthetic vaporisers even of the above-described type have their drawbacks. In this connection it is inherent in the mode of operation of an anaesthetic vaporiser that the vaporiser be used only in a specified orientation. If a conventional vaporiser is displaced from its intended orientation to a substantial degree while charged with liquid anaesthetic there is the likelihood that liquid will find its way into the gas outlet passages of the vaporiser with potentially dire results. Specifically, the hazards associated with such an occurrence may include the administration of a dangerously high concentration of anaesthetic agent as the liquid vaporises, the possible lethal delivery of liquid into a patient's airway, or (in the by-pass case) a dangerous change in the delivered concentration due to the presence of liquid in a part of the vaporiser where it is not intended to be altering the relative resistances to gas flow through the parallel circuits. There have in face been reported cases of fatalities when a free standing vaporiser has been knocked over during use and the resultant surge of liquid from the outlet of the vaporiser has been blown into the patient's airway. The tilting or inversion of a conventional, charged vaporiser even when disconnected from the gas supply is also potentially hazardous, however, and this problem is highlighted by the increasing use of anaesthesia machines which have provision for readily interchanging vaporisers, as explained below.
There are a number of different volatile anaesthetic agents having different medically desirable properties in common usage today. However, the various agents have different vapour pressure characteristics and are effective in different output concentrations, with the result that different vaporisers specifically constructed and calibrated for use with each agent are required. With the advent of anaesthesia machines with port valve fittings which permit the rapid installation and removal of different vaporisers, a given machine can readily be adapted for the administration of any one of the range of available anaesthetic agents and, using the same basic machine, the anaesthetist is accordingly given the freedom to match the correct agent as far as possible to the needs of every patient. Desirable as this practice is, it leads to an increase in the occasions when a charged vaporiser must be carried around from one station to another (e.g. from a storage rack to an anaesthesia machine or vice versa) and fitted in place at the different stations. If in the course of this handling a charged vaporiser is inadvertently inverted or tilted to a substantial degree then, even though there may be no gas flowing through it at the time, the danger exists of it subsequently delivering a higher than intended anaesthetic concentration if the liquid has been permitted to collect in any part of the gas outlet passages.
Heretofore, vaporisers have been available which have `non-spill` properties in that if a vaporiser is tilted or inverted the contained liquid cannot reach the outlet from the vaporising chamber. Such vaporisers have employed relatively large cylindrical vaporising chambers with the gas inlet(s) and outlet(s) from the chamber being placed in the region of its centre of volume. For this `non-spill` approach to be effective, the vaporising chambers of such devices must have internal volumes of many times the free liquid capacity and hence they are unsuitable for use in conjunction with continuous flow anaesthesia machines where they are liable to be subject to fluctuating back pressures from lung ventilators etc. It is a requirement of vaporisers intended for such use that the free volume in the vaporising chamber be as small as possible because the larger the free volume of vapour-saturated gas within the vaporiser the more likely it is that vapour can be carried back to the inlet of the vaporiser by a reverse-flowing pressure wave, despite measures which are designed to minimise the effects of such back pressures. If this back flow occurs the vapour can then flow forward through the by-pass circuit during the next pressure cycle, thereby increasing the anaesthetic concentration delivered by the vaporiser to an uncontrollable degree.